It was a sensational discovery - human footprints
said to be 40,000 years old, preserved by volcanic ash in an
abandoned quarry in Mexico.
The announcement was made in July 2005, and it
created a flurry of excitement. Then it was promptly dismissed by
a second team of researchers who re-dated the rocks at 1.3 million
years old, impossibly ancient to bear human traces. The original
claim did not go away, however.
The first widespread evidence for human occupation
in North America came from the town of Clovis in New Mexico. The
beautiful fluted stone spear points made by the Clovis people are
found on many sites and they date back to around 11,500 BC. They are
believed to have been left by people who descended from those who
crossed a land bridge that once existed between Siberia and Alaska.
But there is an increasing body of evidence for earlier
occupation of the Americas, dating back to a time at which the overland
route through the ice would have been impossible.
The best evidence probably comes from Monte Verde in
Chile and dates back to at least 12,500 BC. But to have reached so far
south by that date, people must have entered the continent earlier still.
There have been many claims of earlier dates, but few have been
substantiated. So the announcement of 40,000 year-old footprints from
Mexico was greeted with scepticism and caution.
It came from a team led by Silvia Gonzalez, a Mexican
working at Liverpool John Moores University (JMU), UK. In 2003, she was
visiting a site to the south of Puebla, about a hundred kilometres
south-east of Mexico City.
It is a dry environment with many small volcanoes and,
in the distance, the smoking peak of Popocatepetl. She was hoping to
find the geological context of deposits that had yielded animal bones
showing possible butchery marks which dated back between 20,000-30,000
The researchers were looking for a vertical section
through the rocks in the side of a small quarry, but it was overgrown
and strewn with debris. As they were about to give up, they noticed that
the floor of the quarry was made of a single layer of hardened volcanic
ash called the xalnene tuff. This stuff looks a bit like a badly asphalted